The present invention relates to an improved circulating, i.e., fast, fluidized bed reactor utilizing a cyclone of turbulent gases in the upper region of the reaction chamber, and to a method of operating the reactor; and, more particularly, to a reactor of this type utilizing cyclone particle separators and to a reactor of this type wherein such cyclone separators are eliminated.
The present invention has specific application, inter alia, to adiabatic fluidized bed combustors, fluidized bed boilers, and fluidized bed gasifiers. As used herein, and in the accompanying claims, "adiabatic combustor" denotes a fluidized bed combustor that does not contain internal cooling means, and "boiler" denotes a fluidized bed combustor that contains internal heat absorption means, in the form of immersed boiler, superheater, evaporator, and/or economizer heat exchange surfaces. The temperature of adiabatic fluidized bed combustors is typically controlled by the use of pressurized air in substantial excess of the stoichiometric amount needed for combustion. On the other hand, fluidized bed boilers require very low excess air, so that heat absorption means are required in the fluidized bed. Fluidized bed gasifiers, in contrast, utilize less than stoichiometric amounts of air.
The state of fluidization in a fluidized bed of solid particles is primarily dependent upon the diameter of the particles and the fluidizing gas velocity. At relatively low fluidizing gas velocities exceeding the minimum fluidizing velocity, e.g., a fluidization number in the range from about 2 to 10, the bed of particles is in what has been termed the "bubbling" regime. Historically, the term "fluidized bed" has denoted operation in the bubbling regime. This fluidization mode is generally characterized by a relatively dense bed having an essentially distinct upper bed surface, with little entrainment, or carry-over, of the bed particles (solids) in the flue gas, so that recycling the solids is generally unnecessary. At higher fluidizing gas velocities, above those of the bubbling regime, the upper surface of the bed becomes progressively diffuse and carry-over of the solids increases, so that recirculation of solids using a particulate separator, e.g., a cyclone separator, becomes necessary in order to preserve a constant solids inventory in the bed.
The amount of solids carry-over depends upon the fluidizing gas velocity and the distance above the bed at which the carry-over occurs. If this distance is above the transfer disengaging height, carry-over is maintained at a constant level, as if the fluidizing gas were "saturated" with solids.
If the fluidizing gas velocity is increased above that of the bubbling regime, the bed then enters what has been termed the "turbulent" regime, and finally, the "fast," i.e., "circulating," regime. If a given solids inventory is maintained in the bed, and the fluidizing gas velocity is increased just above that of the turbulent regime, the bed density drops sharply over a narrow velocity range. Obviously, if a constant solids inventory is to be preserved in the bed, the recirculation, or return, of solids must equal the carry-over at "saturation."
At fluidizing gas velocities below those associated with the aforementioned sharp drop in bed density, the effect upon bed density of returning solids to the fluidized bed at a rate well above the "saturation" carry-over is not marked. The addition of solids to a bed fluidized in either the bubbling or turbulent regime at a rate above the saturation carry-over will simply cause the vessel containing the fluidized bed to fill up continually, while the fluidized density will remain substantially constant. However, at the higher fluidizing gas velocities associated with the fast regime, the fluidized density becomes a marked function of the solids recirculation rate.
Fast fluidized beds afford intimate contact between the high velocity fluidizing gas and a large inventory of solids surface per unit bed volume. Also, slip velocity (i.e., solids-fluidizing gas relative velocity) is relatively high in fast fluidized beds, when compared with that in ordinary fluidized beds. Additionally, the combustion process which takes place in a fast fluidized bed combustor is generally more intense, having a higher combustion rate, than that occurring in traditional fluidized bed combustors. Furthermore, as a result of the high solids recirculation rate in fast fluidized beds, the temperature is essentially uniform over the entire height of such combustors.
The higher combustion reaction rate, compared to that of ordinary fluidized bed combustors, allows the combustion temperature in fast fluidized bed combustors to be significantly reduced. Reduction of the combustion temperature may be accomplished, for example, by inserting heat exchanger tubes in the combustion region. Reducing the combustion temperature leads directly to a reduction in the total cost of constructing fast fluidized bed boilers, since (1) the total boiler heat exchange surface can be reduced, (2) thinner refractory bed liners are required, and (3) smaller cyclone separators can be installed. Moreover, contrary to prior art teachings, wet biomass materials may be combusted at such reduced combustion temperatures.
Notwithstanding the many advantages offered by fast fluidized bed reactors, as enumerated above, the high cost of constructing and maintaining the extremely large external separation cyclones and large diameter standpipe required for recirculation of the entrained solids at the rate necessary to maintain the bed in the fast fluidization regime constitutes a severe economic impediment to widespread commercial utilization of such reactors. In this regard, prior art fast fluidized bed combustors are known which employ heat exchanger tube-lined walls in the entrainment region of the combustor (i.e., parallel to the flow). Such combustors rely primarily on the transfer of radiant heat from gases which typically are heavily laden with solids. Nevertheless, such combustors require an extremely large internal volume. Furthermore, still higher combustion rates are desired in fast fluidized bed boilers, with a concomitant reduction of the combustion temperature, and thus the size of the combustor so as to reduce the cost of construction.
In the past, cyclone combustors which produce a cyclone of turbulent gases within the combustion chamber have been employed for combusting various solid materials, including poor quality coal and vegetable refuse, as disclosed, for example, in "Combustion in Swirling Flows: A Review," N. Syred and J. M. Beer, Combustion and Flame, Vol. 23, pp. 143-201 (1974). Such cyclone combustors do not, however, involve the use of fluidized beds.
Although providing high specific heat release, prior art cyclone combustors suffer the following disadvantages: (1) the size of the usable fuel particles is limited to 0.25 inch (average effective diameter); (2) fuel moisture content is limited to about 3-5%; (3) at close to stoichiometric combustion, there is no means to control combustion temperature below the fusion point; and (4) erosion of refractory linings may occur in some instances.
Although the ordinary fluidized bed incinerator system described in U.S. Pat. No. 4,075,953 to Sowards, for example, is provided with a vortex generator, this system does not exhibit the combustion characteristics associated with conventional prior art cyclone combustors. In particular, the specific heat release is quite low (about 0.2.times.10.sup.6 Kcal per cubic meter per hour) and the Swirl number [defined in terms of combustor input and exit parameters as S=(Input Axial Flux of Angular Momentum)/(De/2.times.Exit Axial Flux of Linear Momentum), where D.sub.e is the combustor exit throat diameter] is no greater than about 0.07.
Likewise, while the conventional combustion furnace described in U.S. Pat. No. 4,159,000 employs tangentially disposed air inlets, it does not achieve the combustion characteristics of conventional cyclone combustors (e.g., it exhibits a lower Reynolds number and lower specific heat release).
In conventional, i.e., non-circulating, fluidized bed reactors for combusting particulate material, the material to be combusted is fed over a bed of granular material, usually sand. In such reactors, it is desirable to be able to vary the amount of particulate material fed to the reactor and, concomitantly, the amount of pressurized air supplied to the reactor over as wide a range as possible. The hydrodynamic turndown ratio of a reactor, which is defined as the ratio of pressurized air flow at maximum reactor load to pressurized air flow at minimum reactor load, is a measure of the ability of a reactor to operate over the extremes of its load ranges. Notwithstanding the need for a fluidized bed reactor with turndown ratios in excess of 2 to 1, so as to improve the ability of the reactor to respond to varying power demands, the prior art has not satisfactorily provided a solution.
By way of example, prior art non-circulating fluidized bed boilers are known which employ an oxidizing fluidized bed for heat generation. In such boilers, relatively high heat releases and heat transfer directly from the fluidized bed material to heat exchange surfaces immersed therein serve to enhance the efficiency of the boiler, thereby reducing the boiler dimensions required to produce the desired thermal output, when compared with traditional boiler designs. Although high heat exchange efficiency is inherent in the operation of such oxidizing fluidized bed boilers, such boilers have a low turndown ratio, requiring a relatively narrow range in the variation of fuel consumption and heat output. These disadvantages have impeded widespread commercialization of such oxidizing fluidized bed boilers.